Magnetic tunnel junction (MTJ) elements serve as magnetoresistive elements. A magnetic tunnel junction element basically has a multilayer structure including a storage layer in which the magnetization direction is changeable, a reference layer in which the magnetization direction is unchangeable, and an insulating layer disposed between the storage layer and the reference layer. The MTJ element is known to have a tunneling magnetoresistive (TMR) effect, and used as a storage element of a memory cell in magnetic random access memories (MRAMs).
An MRAM is a nonvolatile memory in which each MTJ element stores information (“1” or “0”) determined based on the change in relative angle of the magnetizations in its magnetic layers. The magnetization is switched (changed) in a few nanoseconds. This enables high-speed writing and reading of data. Therefore, the MRAMs are expected to be next-generation, high-speed nonvolatile memories. The cell size of the MRAMs may be reduced by employing so-called “spin transfer torque magnetization switching,” which controls magnetization by means of spin polarized current. Reducing the cell size leads to an increase in current density. Thus, the MRAMs with advantages of easy magnetization switching of storage layers, high density, and low power consumption may be obtained.
In improving the density of nonvolatile memories, the magnetoresistive elements should be highly integrated. However, the thermal stability of the ferromagnetic materials of the magnetoresistive elements tends to be reduced as the element size is decreased. Therefore, improvement in the magnetic anisotropy and the thermal stability of the ferromagnetic materials is a problem of the MRAMS.
In order to solve the problem, it has recently been attempted to form MRAMs with perpendicular magnetization MTJ elements, in which the magnetization of a ferromagnetic material is oriented in a direction perpendicular to the film plane. The ferromagnetic materials of the perpendicular magnetization MTJ elements generally have a large crystalline magnetic anisotropy.